Chapter VII 

 REPRODUCTION AND DEVELOPMENT 



Types of Reproduction. One of the most striking and per- 

 haps the most universal distinction between living and non-living 

 objects is that the former are invariably capable at some time during 

 their life history of taking part in producing another living object 

 nearly identical with its parent. We have seen that this is accom- 

 plished in two ways: Either by the independent development of a 

 cell or group of cells (that is, a bud or a fragment) into a new 

 adult organism, or by the development of a zygote, which results 

 when an ovum and a sperm cell, usually derived from separate 

 individuals, have united. The former, agamic reproduction, results 

 in an individual that shows no diflferences from the parent. On the 

 other hand, an animal that results from the development of a zygote 

 may, and usually does, resemble both the parents. 



Agamic Reproduction. Agamic reproduction is very common 

 among both plants and animals. In its simplest form in animals, 

 among the Protozoa, it first involves a division of the cell nucleus, 

 followed by a separation of the cell body into two or more daughter 

 cells, each with the structural and functional characteristics of a 

 young animal. In most Protozoa only two daughter organisms 

 result from each division. In the Sporozoa, however, a considerable 

 number of nuclear divisions precede the division of the parent cell; 

 division results in as many daughter organisms as there are daugh- 

 ter nuclei (Fig. 31). Among Protozoa that form colonies, for 

 example volvox (Fig. 27), the products of a division may not be 



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